The invention relates to a lighting unit comprising an electrodeless low-pressure discharge lamp provided with a discharge vessel which encloses a discharge space containing an ionizable filling in a gastight manner and which has a recessed potion and an enveloping portion which is permeable to radiation generated in the discharge vessel, while the recessed portion is provided with a luminescent layer at a surface facing towards the discharge space and means are present in the recessed portion for achieving and maintaining an electric discharge in the discharge space, which means comprise a member for generating an electric field in the discharge space, and the lighting unit in addition has a supply unit for supplying said means.
The invention also relates to an electrodeless low-pressure discharge lamp for use in the lighting unit.
The invention also relates to a discharge vessel for use in the lighting unit.
The term "electrodeless low-pressure discharge lamp", also referred to as lamp hereinafter, in the present description and claims is understood to mean a low-pressure discharge lamp in which electrodes in the discharge space are absent. It is possible, however, for electrodes to be present outside the discharge space.
A lighting unit as described above is known from U.S. Pat. No. 4,710,678. In the known lighting unit, a supply unit is accommodated in a housing fastened to the discharge vessel of the lamp. The discharge vessel is entirely coated with a luminescent layer at a surface facing towards the discharge space and is provided with a filling comprising mercury. A coil having a primary winding and a secondary winding around a core of soft magnetic material forms means for achieving and maintaining an electric discharge. When the lighting unit is switched on, an ignition voltage is applied across the primary winding of the coil so as to generate an electric field which initiates a discharge. The magnetic field of the coil maintains the discharge during nominal operation. Electromagnetic interference in the surroundings and in the power mains is counteracted by the secondary winding.
A disadvantage of the known lighting unit is that a comparatively high ignition voltage is required for starting a discharge, especially when the unit has been out of operation for a comparatively long time. The requirements to be imposed on the supply unit and the electrical insulation of lamp components are accordingly comparatively high.